The present invention relates to a waterjet assembly for an entrainment waterjet cutting head device.
In an entrainment cutting head of an abrasive waterjet apparatus, water at pressures of up to 6000 bar flows through a collimation tube to a waterjet generating means and creates a waterjet travelling at up to 3 times the speed of sound. The waterjet traverses a chamber between a waterjet generating means and a focus tube and enters a focus tube bore. Abrasive particles carried in a fluid are entrained into the chamber by the waterjet and on into the focus tube bore. Momentum is transferred from a waterjet to abrasive particles in a focus tube bore to produce a cutting jet at a focus tube outlet.
Abrasive is taken to mean abrasive particles of a material such as garnet, olivine or aluminium oxide. Abrasive can be transported in tubing to a cutting head dynamically suspended in airflow or it can be transported essentially statically suspended in water. Abrasive particles essentially statically suspended in water are referred to as abrasive suspension. A collimation tube is taken to mean passaged components upstream of a waterjet generating means whose passage centrelines are essentially collinear with the axis of a waterjet generating means. A waterjet generating means is taken to mean a nozzle or orifice that converts pressurised water into a waterjet.
A prior art entrainment cutting head that entrains abrasive carried in air has a waterjet generating means in the form of an orifice made from ruby, sapphire, natural diamond or monocrystalline diamond. These materials are brittle and spoil and crack if subjected to excessive point or uneven loading. An orifice is sealingly located in the front face of a carrier or is retained in sintered metal within a carrier. Water pressure forces acting on an orifice are transmitted from a waterjet orifice into a substantial body of carrier material downstream of an orifice. A carrier is designed to support a waterjet orifice and prevent an orifice acting as structural element.
To generate a high quality waterjet traveling at 2 to 3 times the speed of sound it has been found necessary to have a 100 to 1 or so contraction in area from the front face of a waterjet orifice carrier to a waterjet orifice bore. Water pressure force on the front face of a carrier is transmitted through a carrier to a cutting head body.
A threaded connection on a cutting head body is used to exert substantial force to align and to cause plastic deformation of asperities and minor errors on mating faces between a collimation tube and a waterjet orifice carrier to form a metal to metal seal. Care is taken to ensure the sealing force is not transmitted to a waterjet generating means and in particular does not affect the alignment of a waterjet generating means.
Strong re-circulation flows occur in the chamber between a waterjet orifice and a focus tube of prior art cutting heads that entrain abrasive carried in air. These re-circulation flows carry abrasive particles that erode a waterjet orifice and its carrier. Air entrainment commences as soon as a waterjet separates from an orifice edge, and this entrainment carries particles up inside the bore of an orifice when a waterjet is present to reach the region where water separates from an orifice edge to form a waterjet. Closing a valve to stop water flow through a waterjet orifice causes extreme cavitation downstream of a valve with cavitation vapour cavities collapsing on final closure of a valve. The collapse of cavities reverses flow through an orifice and can carry particles present in an orifice bore upstream of an orifice. A waterjet orifice edge can be damaged when water flow is re-started with particles upstream of an orifice. To minimise wear and damage from particles reaching an orifice a separation distance of 50 or so waterjet diameters is used between a waterjet orifice and a focus tube in prior art cutting heads. A distance of 50 or so waterjet diameters allows space for a substantial carrier to support a waterjet orifice.
To generate abrasive waterjets with diameters less than 200 microns or so that are required for micro machining it has been found necessary to substantially reduce the distance, in terms of waterjet diameters, between a waterjet generating means and a focus tube. Substantially reducing the distance between a waterjet generating means and a focus tube does not allow adequate space to support a waterjet generating means on its outlet face.
Except for a cutting head described in International Patent Application PTC/GB2006/004084 that has a waterjet nozzle attached by its upstream face to the downstream face of a carrier, prior art cutting heads that entrain abrasive suspensions are not suitable for precision machining because of poor cutting performance. Poor cutting performance results from energy dissipation by excessive turbulent mixing before a waterjet enters a focus tube and by turbulent energy dissipation as the water jet expands within the focus tube bore to fill the focus bore. Excessive turbulent energy dissipation before a focus tube can be avoided by substantially reducing the separation distance between a waterjet generating means and a focus tube compared to prior art. Preventing energy dissipation caused by a waterjet expanding to fill the full cross section of a focus tube bore requires a state of super cavitation to exist between the outlet of a waterjet generating means and a focus tube outlet. Maintaining a state of super cavitation requires the separation distance between the waterjet generating means and the focus tube to be made as small as practical, whilst allowing sufficient flow area for abrasive in suspension to enter into the focus tube inlet. It also requires that the waterjet be extremely accurate aligned along the axis of the focus tube bore.
In prior art cutting heads the alignment of waterjet generating means and focus tube centrelines depends on tolerances on at least four machining operations involving centreline locations:    a) the centreline of the waterjet generating means relative to a reference diameter on a carrier,    b) the centreline of the location diameter in a bore in a cutting head body that locates a carrier in the cutting head body,    c) the centreline location within a cutting head body of a bore for a focus tube, and    d) the location of a focus tube bore centreline relative to the outside diameter of a focus tube.
An important factor in the rapid growth of the market for abrasive waterjet machining systems was the development of entrainment cutting heads that replaced troublesome manual alignment of a focus tube bore with a waterjet by alignment through tight control over tolerances on cutting head components. Reducing cutting jet diameters to below 200 microns, to carry out micro machining, requires the centreline of a waterjet generating means to be aligned along the centreline of a focus tube within microns.
Workpiece cut surface tolerances depend on the circularity of a focus tube bore. A waterjet that is not aligned along a focus tube causes uneven and increased focus tube wear. For micromachining, a cutting head motion system desirably positions a cutting with a repeatability of 3 microns or so and a cut accuracy of 10 microns or so is desirable. This level of cut accuracy can only be met if focus tube bore wear is even around the bore circumference.
Consistently achieving a waterjet alignment within microns along the centreline of a focus tube for micro machining requires minimising the number of toleranced dimensions affecting centreline alignment.